Programmable low dropout regulators with fast transient response when programming output voltage

ABSTRACT

Apparatus and methods for programmable low dropout regulators for radio frequency (RF) electronics are provided herein. In certain configurations, a power amplifier system includes a multi-stage power amplifier that amplifies a radio frequency signal and includes a driver stage and an output stage. The power amplifier system further includes an envelope tracker that controls a supply voltage of the output stage in relation to an envelope of the radio frequency signal, and a programmable low dropout regulator that includes an output that provides a programmable supply voltage to the driver stage. The programmable low dropout regulator includes an output capacitor electrically connected to the output, a regulation transistor having a drain electrically connected to the output, and an alternative discharge circuit that discharges the output capacitor in response to programming the programmable supply voltage from a high voltage level to a low voltage level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/784,757, filed Oct. 16, 2017 and titled “PROGRAMMABLE LOW DROPOUTREGULATORS WITH FAST TRANSIENT RESPONSE WHEN PROGRAMMING OUTPUTVOLTAGE,” which is a continuation of U.S. application Ser. No.15/219,915, filed Jul. 26, 2016 and titled “APPARATUS AND METHODS FORPROGRAMMABLE LOW DROPOUT REGULATORS FOR RADIO FREQUENCY ELECTRONICS,”which claims the benefit of priority under 35 U.S.C. § 119(e) of U.S.Provisional Patent Application No. 62/206,120, filed Aug. 17, 2015 andtitled “APPARATUS AND METHODS FOR PROGRAMMABLE LOW DROPOUT REGULATORSFOR RADIO FREQUENCY ELECTRONICS,” each of which are herein incorporatedby reference in their entireties.

BACKGROUND Field

Embodiments of the invention relate to electronic systems, and inparticular, to programmable low dropout regulators for radio frequency(RF) electronics.

Description of the Related Technology

Power amplifiers can be included in mobile devices to amplify RF signalsfor transmission via antennas. It can be important to manage the powerof RF signal transmissions to prolong battery life and/or provide asuitable transmit power level.

One technique for improving efficiency of a power amplifier is averagepower tracking (APT), in which a DC-to-DC converter is used to generatea supply voltage for a power amplifier based on the power amplifier'saverage output power. Another technique for improving efficiency of apower amplifier is envelope tracking, in which a supply voltage of thepower amplifier is controlled in relation to the envelope of the RFsignal. Thus, when a voltage level of the envelope of the RF signalincreases the voltage level of the power amplifier supply voltage can beincreased. Likewise, when the voltage level of the envelope of the RFsignal decreases the voltage level of the power amplifier supply voltagecan be decreased to reduce power consumption.

SUMMARY

In certain embodiments, the present disclosure relates to a programmablelow dropout regulator. The programmable low dropout regulator includesan error amplifier, a regulation field-effect transistor, a feedbackcircuit, an output capacitor, and an alternative discharge circuit. Theerror amplifier includes an output and a first input. The regulationfield-effect transistor includes a gate electrically connected to theoutput of the error amplifier and a drain electrically connected to anoutput node configured to provide an output voltage that isprogrammable. The feedback circuit is electrically connected between theoutput node and a first voltage, and is configured to provide a feedbacksignal to the first input of the error amplifier. The output capacitoris electrically connected to the output node, and the alternativedischarge circuit is configured to discharge the output capacitor inresponse to programming the output voltage from a high voltage level toa low voltage level.

In some embodiments, the alternative discharge circuit includes acomparator configured to compare the output voltage to a triggervoltage, and to selectively activate an electrical path from the outputnode to the first voltage based on the comparison. In a number ofembodiments, the alternative discharge circuit further includes adischarge field-effect transistor including a source electricallyconnected to the first voltage and a gate electrically connected to anoutput of the comparator. In various embodiments, the alternativedischarge circuit further includes a discharge control circuitelectrically connected between the output node and the drain of theregulation field-effect transistor. According to some embodiments, thealternative discharge circuit further includes a trigger voltagegeneration circuit configured to generate the trigger voltage, and thetrigger voltage generation circuit includes one or more resistors and acurrent source that provides a current that flows through the one ormore resistors to generate the trigger voltage. In accordance withseveral embodiments, the low dropout regulator is configured to receivea control signal operable to control a voltage level of the outputvoltage, and the alternative discharge circuit is configured to adjust avoltage level of the trigger voltage based on the control signal.

According to various embodiments, the feedback circuit includes a firstresistor and a second resistor electrically connected in series betweenthe output node and the first voltage, and the first resistor has aprogrammable resistance operable to change a voltage level of the outputvoltage.

In a number of embodiments, the programmable low dropout regulatorfurther includes a reference voltage generator configured to provide areference voltage to a second input of the error amplifier.

In certain embodiments, the present disclosure relates to a poweramplifier system. The power amplifier system includes a multi-stagepower amplifier including a driver stage, and a programmable low dropoutregulator including an output that powers the driver stage with anoutput voltage that is programmable. The programmable low dropoutregulator includes an error amplifier, a regulation field-effecttransistor including a gate electrically connected to an output of theerror amplifier and a drain electrically connected to the output of theprogrammable low dropout regulator, a feedback circuit configured toprovide feedback to a first input of the error amplifier based on avoltage level of the output voltage, an output capacitor electricallyconnected to the output of the programmable low dropout regulator, andan alternative discharge circuit configured to discharge the outputcapacitor in response to programming the output voltage from a highvoltage level to a low voltage level.

In some embodiments, the power amplifier system further includes atransceiver configured to generate a control signal operable to controlthe voltage level of the output voltage. According to severalembodiments, the programmable low dropout regulator further includes areference voltage generator configured to generate a reference voltagefor a second input of the error amplifier, and the reference voltagegenerator is configured to receive the control signal. In variousembodiments, the feedback circuit includes a resistor having aprogrammable resistance controlled by the control signal.

In accordance with a number of embodiments, the alternative dischargecircuit includes a comparator configured to compare the output voltageto a trigger voltage, and to selectively activate an electrical pathfrom the output of the programmable low dropout regulator to a firstvoltage based on the comparison. In various embodiments, the electricalpath includes a discharge control circuit and a discharge field-effecttransistor electrically connected in series between the output of theprogrammable low dropout regulator and the first voltage.

In some embodiments, the multi-stage power amplifier further includes anoutput stage, and the power amplifier system further includes a DC-to-DCconverter that powers the output stage.

In various embodiments, the multi-stage power amplifier further includesan output stage, and the power amplifier system further includes anenvelope tracker that powers the output stage.

In certain embodiments, the present disclosure relates to a method ofvoltage regulation in a programmable low dropout regulator. The methodincludes controlling a gate of a regulation field-effect transistorusing an error amplifier, regulating an output voltage provided at anoutput node using the regulation field-effect transistor, generating afeedback signal for a first input of the error amplifier based on avoltage level of the output voltage using a feedback circuit,stabilizing the output voltage using an output capacitor, anddischarging the output capacitor using an alternative discharge circuitin response to programming the output voltage from a high voltage levelto a low voltage level.

In some embodiments, the method further includes powering a driver stageof a power amplifier using the output voltage.

In various embodiments, the method further includes comparing the outputvoltage of the low dropout regulator to a trigger voltage, andselectively activating an electrical path from the output node to afirst voltage through the alternative discharge circuit based on thecomparison. According to several embodiments, the method furtherincludes programming the voltage level of the output voltage using acontrol signal, and adjusting a voltage level of the trigger voltagebased on the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a power amplifier module for amplifyinga radio frequency (RF) signal.

FIG. 2 is a schematic diagram of an example wireless device.

FIG. 3 is a schematic diagram of one embodiment of a power amplifiersystem.

FIG. 4A is a schematic diagram of another embodiment of a poweramplifier system.

FIG. 4B is a schematic diagram of another embodiment of a poweramplifier system.

FIG. 4C is a schematic diagram of another embodiment of a poweramplifier system.

FIG. 5A is a schematic diagram of one embodiment of a low dropout (LDO)regulator.

FIG. 5B is a schematic diagram of another embodiment of an LDOregulator.

FIG. 6 is a schematic diagram of another embodiment of an LDO regulator.

FIG. 7A is a schematic diagram of one embodiment of a packaged module.

FIG. 7B is a schematic diagram of a cross-section of the packaged moduleof FIG. 7A taken along the lines 7B-7B.

FIG. 8A is a graph of one example of laboratory data of output voltageversus time for an LDO regulator.

FIG. 8B is a graph of another example of laboratory data of outputvoltage versus time for an LDO regulator.

DETAILED DESCRIPTION OF EMBODIMENTS

The headings provided herein, if any, are for convenience only and donot necessarily affect the scope or meaning of the claimed invention.

A power management system can be used to generate a supply voltage thathas a programmable voltage level. For example, a power management systemcan be used to generate a programmable supply voltage for a poweramplifier. Additionally, the voltage level of the power amplifier supplyvoltage can be changed over time to improve the power amplifier's poweradded efficiency (PAE).

A transient response of the power management system can be important.For example, in wireless device applications, a supply voltage generatedby a power management system may be specified to settle to a programmedvoltage level in less than a certain amount of time, for instance, about5 μs.

A programmable low dropout (LDO) regulator can be used to generate apower amplifier supply voltage for one or more stages of a poweramplifier, such as an input or driver stage. In such configurations, itis desirable for the LDO regulator to have a low quiescent current toprolong the wireless device's battery life.

An LDO regulator can include an error amplifier, a reference voltagegenerator that generates a reference voltage for the error amplifier'snon-inverting input, a regulation field-effect transistor (FET) having agate controlled by the error amplifier's output and a drain thatprovides voltage regulation to the LDO regulator's output, an outputcapacitor electrically connected to the LDO regulator's output, and afeedback circuit that provides feedback from the LDO regulator's outputto the error amplifier's inverting input. To achieve low quiescentcurrent, the feedback circuit can be implemented with a relatively highresistance, for instance, 100 kΩ or more. To provide stability, the LDOregulator can also include a relatively large output capacitor, forinstance, 0.1 μF or more.

When an LDO regulator is implemented with a relatively high resistancefeedback circuit and a relatively large output capacitor, the LDOregulator can suffer from a large resistor-capacitor (RC) time constantassociated with changing the programmable output voltage of the LDOregulator from a high voltage level to a low voltage level. Forinstance, the LDO regulator may have an RC time constant of 10 μs to 100μs or more in such configurations. However, an LDO regulator with alarge RC time constant may be unsuitable for radio frequency (RF)systems specified to operate using a programmable supply voltage that isspecified to settle to a programmed voltage level relatively quickly.

The RC time constant of an LDO regulator may be further constrained by aneed to maintain a stability of the LDO regulator's feedback loop overvariation in temperature, process, supply voltage, and/or loadconditions. For instance, a dominant pole of the LDO regulator'stransfer function may change position in frequency with variation inload current, and thus a large output capacitor may be used to providefrequency compensation. Thus, the output capacitor may have a relativelylarge capacitance value to achieve loop stability.

Accordingly, an LDO regulator can suffer from a trade-off between lowpower consumption (associated with a large feedback resistance), robuststability (associated with a large output capacitance), and fasttransient response (associated with small feedback resistance and smalloutput capacitance).

Apparatus and methods for programmable LDO regulators for RF electronicsare provided herein. In certain configurations, an LDO regulator forgenerating a programmable output voltage includes a regulation FEThaving a drain electrically connected to the LDO regulator's output, anerror amplifier that controls a gate of the regulation FET, a feedbackcircuit that provides a feedback signal to an inverting input of theerror amplifier, an output capacitor electrically connected to the LDOregulator's output, and an alternative discharge circuit. When theoutput voltage of the LDO regulator is programmed from a high voltagelevel to a low voltage level, the alternative discharge circuitactivates to discharge the output capacitor to improve the LDOregulator's transient response.

By configuring the LDO regulator in this manner, the LDO regulator canhave a transient response from high output voltage to low output voltagethat is not constrained by an RC time constant associated with aresistance of the feedback circuit and a capacitance of the outputcapacitor. Thus, the LDO regulator achieves rapid output voltagetransition when being programmed from a high output voltage level to alow output voltage level, while maintaining the benefits of robuststability and low quiescent current. The LDO regulator also achievesrapid output voltage transition when being programmed from a low outputvoltage level to a high output voltage level, since the regulation FETcan be used to charge the output capacitor for such a transition.

In certain configurations, the alternative discharge circuit includes adetection circuit such as a comparator that compares the LDO regulator'soutput voltage to a target voltage. In certain implementations, thetarget voltage is generated using a replica of a portion of the LDOregulator, such as a replica of all or part of the LDO regulator'sfeedback circuit. Additionally, the detection circuit can selectivelyactivate the alternative discharge circuit to discharge the outputcapacitor when a voltage difference between the LDO regulator's outputvoltage and the target voltage is relatively large. However, when thevoltage difference between the LDO regulator's output voltage and thetarget voltage is relatively small, the detection circuit can deactivatethe alternative discharge circuit to inhibit the alternative dischargecircuit from generating overshoot, undershoot and/or glitch in the LDOregulator's output voltage. Thus, the alternative discharge circuit candeactivate and provide a smooth hand-off of control of the outputvoltage to the LDO regulator's control loop.

FIG. 1 is a schematic diagram of a power amplifier module (PAM) 10 foramplifying an RF signal. The illustrated power amplifier module 10amplifies an RF signal (RF_IN) to generate an amplified RF signal(RF_OUT). The power amplifier module 10 can include one or more LDOregulators implemented using one or more features of the presentdisclosure.

FIG. 2 is a schematic block diagram of an example wireless or mobiledevice 11. The wireless device 11 can include one or more LDO regulatorsimplemented using one or more features of the present disclosure.

The example wireless device 11 depicted in FIG. 2 can represent amulti-band and/or multi-mode device such as a multi-band/multi-modemobile phone. By way of examples, Global System for Mobile (GSM)communication standard is a mode of digital cellular communication thatis utilized in many parts of the world. GSM mode mobile phones canoperate at one or more of four frequency bands: 850 MHz (approximately824-849 MHz for Tx, 869-894 MHz for Rx), 900 MHz (approximately 880-915MHz for Tx, 925-960 MHz for Rx), 1800 MHz (approximately 1710-1785 MHzfor Tx, 1805-1880 MHz for Rx), and 1900 MHz (approximately 1850-1910 MHzfor Tx, 1930-1990 MHz for Rx). Variations and/or regional/nationalimplementations of the GSM bands are also utilized in different parts ofthe world.

Code division multiple access (CDMA) is another standard that can beimplemented in mobile phone devices. In certain implementations, CDMAdevices can operate in one or more of 800 MHz, 900 MHz, 1800 MHz and1900 MHz bands, while certain W-CDMA and Long Term Evolution (LTE)devices can operate over, for example, 22 or more bands.

One or more features of the present disclosure can be implemented in theforegoing example modes and/or bands, and in other communicationstandards. For example, 802.11, 2G, 3G, 4G, LTE, and Advanced LTE arenon-limiting examples of such standards. To increase data rates, thewireless device 11 can operate using complex modulated signals, such as64 QAM signals.

In certain embodiments, the wireless device 11 can include switches 12,a transceiver 13, an antenna 14, power amplifiers 17 a, 17 b, a controlcomponent 18, a computer readable medium 19, a processor 20, a battery21, and a power management system 30.

The transceiver 13 can generate RF signals for transmission via theantenna 14. Furthermore, the transceiver 13 can receive incoming RFsignals from the antenna 14.

It will be understood that various functionalities associated with thetransmission and receiving of RF signals can be achieved by one or morecomponents that are collectively represented in FIG. 2 as thetransceiver 13. For example, a single component can be configured toprovide both transmitting and receiving functionalities. In anotherexample, transmitting and receiving functionalities can be provided byseparate components.

Similarly, it will be understood that various antenna functionalitiesassociated with the transmission and receiving of RF signals can beachieved by one or more components that are collectively represented inFIG. 2 as the antenna 14. For example, a single antenna can beconfigured to provide both transmitting and receiving functionalities.In another example, transmitting and receiving functionalities can beprovided by separate antennas. In yet another example, different bandsassociated with the wireless device 11 can operate using differentantennas.

In FIG. 2, one or more output signals from the transceiver 13 aredepicted as being provided to the antenna 14 via one or moretransmission paths 15. In the example shown, different transmissionpaths 15 can represent output paths associated with different bandsand/or different power outputs. For instance, the two example poweramplifiers 17 a, 17 b shown can represent amplifications associated withdifferent power output configurations (e.g., low power output and highpower output), and/or amplifications associated with different bands.Although FIG. 2 illustrates a configuration using two transmission paths15 and two power amplifiers 17 a, 17 b, the wireless device 11 can beadapted to include more or fewer transmission paths 15 and/or more orfewer power amplifiers.

In FIG. 2, one or more detected signals from the antenna 14 are depictedas being provided to the transceiver 13 via one or more receiving paths16. In the example shown, different receiving paths 16 can representpaths associated with different bands. For example, the four examplereceiving paths 16 shown can represent quad-band capability that somewireless devices are provided with. Although FIG. 2 illustrates aconfiguration using four receiving paths 16, the wireless device 11 canbe adapted to include more or fewer receiving paths 16.

To facilitate switching between receive and transmit paths, the switches12 can be configured to electrically connect the antenna 14 to aselected transmit or receive path. Thus, the switches 12 can provide anumber of switching functionalities associated with operation of thewireless device 11. In certain embodiments, the switches 12 can includea number of switches configured to provide functionalities associatedwith, for example, switching between different bands, switching betweendifferent power modes, switching between transmission and receivingmodes, or some combination thereof. The switches 12 can also beconfigured to provide additional functionality, including filteringand/or duplexing of signals.

FIG. 2 shows that in certain embodiments, a control component 18 can beprovided for controlling various control functionalities associated withoperations of the switches 12, the power amplifiers 17 a, 17 b, thepower management system 30, and/or other operating components.

In certain embodiments, a processor 20 can be configured to facilitateimplementation of various processes described herein. The processor 20can implement various computer program instructions. The processor 20can be a general purpose computer, special purpose computer, or otherprogrammable data processing apparatus.

In certain embodiments, these computer program instructions may also bestored in a computer-readable memory 19 that can direct the processor 20to operate in a particular manner, such that the instructions stored inthe computer-readable memory 19.

The illustrated wireless device 11 also includes the power managementsystem 30, which can be used to provide power amplifier supply voltagesto one or more of the power amplifiers 17 a, 17 b. For example, thepower management system 30 can be configured to change the supplyvoltages provided to the power amplifiers 17 a, 17 b to improveefficiency, such as power added efficiency (PAE). The power managementsystem 30 can be used to provide average power tracking (APT) and/orenvelope tracking (ET). Furthermore, as will be described in detailfurther below, the power management system 30 can include one or moreLDO regulators used to generate power amplifier supply voltages for oneor more stages of the power amplifiers 17 a, 17 b. In the illustratedimplementation, the power management system 30 is controlled using apower control signal generated by the transceiver 13. In certainconfigurations, the power control signal is provided by the transceiver13 to the power management system 30 over an interface, such as a serialperipheral interface (SPI) or Mobile Industry Processor Interface (MIN).

In certain configurations, the wireless device 11 may operate usingcarrier aggregation. Carrier aggregation can be used for both FrequencyDivision Duplexing (FDD) and Time Division Duplexing (TDD), and may beused to aggregate a plurality of carriers or channels, for instance upto five carriers. Carrier aggregation includes contiguous aggregation,in which contiguous carriers within the same operating frequency bandare aggregated. Carrier aggregation can also be non-contiguous, and caninclude carriers separated in frequency within a common band or indifferent bands.

FIG. 3 is a schematic block diagram of one example of a power amplifiersystem 26. The illustrated power amplifier system 26 includes theswitches 12, the antenna 14, a directional coupler 24, a powermanagement system 30, a power amplifier bias circuit 31, a poweramplifier 32, and a transceiver 33. The illustrated transceiver 33includes a baseband processor 34, an I/Q modulator 37, a mixer 38, andan analog-to-digital converter (ADC) 39. Although not illustrated inFIG. 3 for clarity, the transceiver 33 can include circuitry associatedwith receiving signals over one or more receive paths.

The baseband signal processor 34 can be used to generate an in-phase (I)signal and a quadrature-phase (Q) signal, which can be used to representa sinusoidal wave or signal of a desired amplitude, frequency, andphase. For example, the I signal can be used to represent an in-phasecomponent of the sinusoidal wave and the Q signal can be used torepresent a quadrature-phase component of the sinusoidal wave, which canbe an equivalent representation of the sinusoidal wave. In certainimplementations, the I and Q signals can be provided to the I/Qmodulator 37 in a digital format. The baseband processor 34 can be anysuitable processor configured to process a baseband signal. Forinstance, the baseband processor 34 can include a digital signalprocessor, a microprocessor, a programmable core, or any combinationthereof. Moreover, in some implementations, two or more basebandprocessors 34 can be included in the power amplifier system 26.

The I/Q modulator 37 can be configured to receive the I and Q signalsfrom the baseband processor 34 and to process the I and Q signals togenerate an RF signal. For example, the I/Q modulator 37 can includedigital-to-analog converters (DACs) configured to convert the I and Qsignals into an analog format, mixers for upconverting the I and Qsignals to RF, and a signal combiner for combining the upconverted I andQ signals into an RF signal suitable for amplification by the poweramplifier 32. In certain implementations, the I/Q modulator 37 caninclude one or more filters configured to filter frequency content ofsignals processed therein.

The power amplifier bias circuit 31 receives a bias control signal fromthe transceiver 33, and generates one or more bias signals for the poweramplifier 32. In the illustrated configuration, the power amplifier biascircuit 31 generates a first bias signal BIAS1 for biasing a driverstage of the power amplifier 32 and a second bias signal BIAS2 forbiasing an output stage of the power amplifier 32. The bias signalsBIAS1, BIAS2 can include current and/or voltage signals, and can beused, for example, to bias bases of bipolar transistors and/or gates offield-effect transistors associated with the power amplifier's stages.In certain configurations, the transceiver 33 can control the biassignals generated by the power amplifier bias circuit 31 to enhance thePAE of the power amplifier system 26. In one embodiment, the transceiver33 controls each of the first and second bias signals BIAS1, BIAS2 toone of a multiple settings based on at least one of a frequency band ofoperation or a power mode (for example, high power mode, medium powermode, or low power mode).

The power amplifier 32 can receive the RF signal from the I/Q modulator37 of the transceiver 33, and when enabled can provide an amplified RFsignal to the antenna 14 via the switches 12. The directional coupler 24can be positioned between the output of the power amplifier 32 and theinput of the switches 12, thereby allowing an output power measurementof the power amplifier 32 that does not include insertion loss of theswitches 12. However, other configurations of power measurement arepossible.

In the illustrated configuration, the sensed output signal from thedirectional coupler 24 is provided to the mixer 38, which multiplies thesensed output signal by a reference signal of a controlled frequency.The mixer 38 operates to generate a downshifted signal by downshiftingthe sensed output signal's frequency content. The downshifted signal canbe provided to the ADC 39, which can convert the downshifted signal to adigital format suitable for processing by the baseband processor 34. Byincluding a feedback path between the output of the power amplifier 32and the baseband processor 34, the baseband processor 34 can beconfigured to dynamically adjust the I and Q signals to optimize theoperation of the power amplifier system 26. For example, configuring thepower amplifier system 26 in this manner can aid in controlling the PAEand/or linearity of the power amplifier 32. However, otherconfigurations of power control can be used.

The power management system 30 receives a power control signal from thetransceiver 33, and generates one or more power amplifier supplyvoltages for the power amplifier 32. In the illustrated configuration,the power management system 30 generates a first power amplifier supplyvoltage V_(CC1) for powering a driver stage of the power amplifier 32and a second power amplifier supply voltage V_(CC2) for powering anoutput stage of the power amplifier 32. In certain configurations, thetransceiver 33 can control the voltage levels of the power amplifiersupply voltages V_(CC1), V_(CC2) to enhance the power amplifier system'sPAE.

The power management system 30 illustrates another example of an RFsystem that can include one or more LDO regulators implemented using oneor more features of the present disclosure.

FIG. 4A is a schematic diagram of another embodiment of a poweramplifier system 50. The power amplifier system 50 includes a poweramplifier 60 and a power management system 61.

The power amplifier 60 includes a driver stage bipolar transistor 51, anoutput stage bipolar transistor 52, an input matching circuit 53, aninterstage matching circuit 54, an output matching circuit 55, a driverstage inductor or choke 57, and an output stage inductor or choke 58.

As shown in FIG. 4A, the power amplifier 60 receives an RF input signalRF_IN, which is amplified using the driver stage bipolar transistor 51.A collector of the driver stage bipolar transistor 51 generates anamplified RF signal, which is provided to a base of the output stagebipolar transistor 52. The output stage bipolar transistor 52 furtheramplifies the amplified RF signal to generate the RF output signalRF_OUT. As shown in FIG. 4A, the emitters of the driver stage bipolartransistor 51 and output stage bipolar transistor 52 are electricallyconnected to a first voltage V₁, which can be, for example, a groundvoltage.

As will be appreciated by persons having ordinary skill in the art, theinput matching circuit 53, the interstage matching circuit 54, and theoutput matching circuit 55 provide impedance matching, thereby enhancingRF performance.

As shown in FIG. 4A, the power amplifier 60 receives a first bias signalBIAS1 for biasing a base of the driver stage bipolar transistor 51, anda second bias signal BIAS2 for biasing a base of the output stagebipolar transistor 52. The first and second bias signals BIAS1, BIAS2can be generated by a power amplifier bias circuit, such as the poweramplifier bias circuit 31 of FIG. 3.

The power amplifier 60 of FIG. 4A is powered using a first poweramplifier supply voltage V_(CC1) and a second power amplifier supplyvoltage V_(CC2). The driver stage supply inductor 57 is electricallyconnected between the first power amplifier supply voltage V_(CC1) andthe collector of the driver stage bipolar transistor 51, and the outputstage supply inductor 58 is electrically connected between the secondpower amplifier supply voltage V_(CC1) and the collector of the outputstage bipolar transistor 52. The driver stage supply inductor 57 and theoutput stage supply inductor 58 can aid in powering the power amplifier60, while providing impedance sufficient to block RF signals generatedby the power amplifier 60 from reaching the first and second power highsupply voltages V_(CC1), V_(CC2).

Although the illustrated power amplifier 60 includes two stages, otherconfigurations are possible, including, for example, power amplifiersincluding three or more stages. Although the illustrated power amplifier60 is implemented using bipolar transistors, the teachings herein arealso applicable to field-effect transistor configurations.

The power management system 61 includes an LDO regulator 62 and aDC-to-DC converter 63. As shown in FIG. 4A, the LDO regulator 62receives a first power control signal CTL1 and generates the first poweramplifier supply voltage V_(CC1), which is used to power the driverstage bipolar transistor 51, in this embodiment. Additionally, theDC-to-DC converter 63 receives a second power control signal CTL2 andgenerates the second power amplifier supply voltage V_(CC2), which isused to power the output stage bipolar transistor 52. In certainconfigurations, the first and second power controls signals CTL1, CTL2are generated by a transceiver, such as the transceiver 33 of FIG. 3.Although the first and second power controls signals CTL1, CTL2 areillustrated as a separate control signals, in certain configurations,the LDO regulator 62 and the DC-to-DC converter 63 can be programmedover a common interface.

The illustrated power amplifier system 50 includes a multi-stage poweramplifier 60 including a driver stage that is powered using the LDOregulator 62 and an output stage that is powered using a DC-to-DCconverter 63. However, other configurations are possible, including, butnot limited to, implementations in which the LDO regulator 62 providespower to a power amplifier stage other than a driver stage. The DC-to-DCconverter 63 can operate using, for instance, average power tracking(APT) to achieve high efficiency for the power amplifier system. Toprovide a further enhancement to the power amplifier system'sefficiency, the power amplifier 60 includes a driver stage that ispowered using the LDO regulator 62. The LDO regulator 62 generates aprogrammable voltage that changes over time to increase the efficiencyof the power amplifier system 50.

Although the DC-to-DC converter 63 can have a higher efficiency than theLDO regulator 62, the illustrated power amplifier system 50 can have alower cost and/or size relative to a power amplifier system implementedusing multiple DC-to-DC converters for generating the first and secondpower amplifier supply voltages V_(CC1), V_(CC2). Thus, the illustratedconfiguration advantageously provides an improvement in PAE bycontrolling the voltage level of the driver stage's power amplifiersupply voltage, while avoiding the cost and/or size increase associatedwith using a DC-to-DC converter to generate the driver stage's poweramplifier supply voltage. The illustrated configuration advantageouslyprovides a performance improvement relative to a configuration using afixed supply voltage to power the power amplifier's driver stage.

The LDO regulator 62 is implemented using an alternative dischargecircuit as described herein. The alternative discharge circuit allowsthe LDO regulator to have a fast transient response when programming thefirst power amplifier supply voltage V_(CC1) from a high voltage levelto a low voltage level using the first power control signal CTL1. Incontrast, a conventional LDO regulator has a transient response that istoo slow to provide a programmable supply voltage for a power amplifier.

Accordingly, the illustrated power amplifier system 50 advantageouslyincludes an LDO regulator for generating a power amplifier supplyvoltage for a driver stage of a power amplifier. Configuring the poweramplifier system 50 in this manner achieves an increase in PAE whilehaving a relatively small impact on cost and/or size of the poweramplifier system 60.

Powering a driver stage of a power amplifier system using an LDOregulator can provide a number of advantages. For instance, a poweramplifier system can include an output stage that is powered using ahighly efficient switching regulator. Additionally, the LDO regulatorcan be programmed to control a voltage at the collector of a driverstage transistor to provide analog pre-distortion in conjunction withthe switching regulator of the output stage. By providing analogpre-distortion using the LDO regulator, the output stage can operatewith relatively low DC current, thereby pushing operation of the outputstage to deeper AB class and achieving higher efficiency. The resultinggain expansion, which degrades linearity, can be compensated by thedriver stage via the programmable LDO regulator. The analogpre-distortion achieved through the programming of the supply voltage ofthe driver stage can be used to provide a substantially flat AM-AMcharacteristic, which leads to efficient linear operation of the poweramplifier over different power levels.

FIG. 4B is a schematic diagram of another embodiment of a poweramplifier system 65. The power amplifier system 65 includes a poweramplifier 60 and a power management system 66.

The power amplifier system 65 of FIG. 4B is similar to the poweramplifier system 50 of FIG. 4A, except that the power amplifier system65 of FIG. 4B includes a different implementation of a power managementsystem 66. In particular, the power management system 66 of FIG. 4B issimilar to the power management system 61 of FIG. 6A, except that thepower management system 66 omits the DC-to-DC converter 63 in favor ofincluding an envelope tracker 67.

Additional details of the power amplifier system 65 can be as describedearlier.

FIG. 4C is a schematic diagram of another embodiment of a poweramplifier system 75. The power amplifier system 75 includes a poweramplifier 70 and a power management system 61.

The power amplifier system 75 of FIG. 4C is similar to the poweramplifier system 50 of FIG. 4A, except that the power amplifier system75 of FIG. 4C includes a different implementation of a power amplifier70. In particular, the power amplifier 70 of FIG. 4C is similar to thepower amplifier 60 of FIG. 6A, except that the power amplifier 70 omitsthe driver stage bipolar transistor 51 and the output stage bipolartransistor 52 in favor of including a driver stage FET 71 and an outputstage FET 72, respectively.

Additional details of the power amplifier system 75 can be as describedearlier.

Although FIGS. 4A-4C illustrated specific embodiments of power amplifiersystems, the teachings herein are applicable to a wide variety of poweramplifier systems, including, but not limited to, power amplifiersystems including different implementations of power management systemsand/or different implementations of power amplifiers. Moreover, theteachings herein are applicable not only to LDO regulators used toprovide power to power amplifiers, but also to LDO regulators used toprovide power to other types of electronic circuitry.

FIG. 5A is a schematic diagram of one embodiment of an LDO regulator 80.The LDO regulator 80 includes an error amplifier 101, a regulation FET102, a feedback circuit 83, an alternative discharge circuit 84, areference voltage generator 105, and an output capacitor 106. The LDOregulator 80 has an output that provides an output voltage V_(CC1). AnLDO regulator's output is also referred to herein as an output node. Thevoltage level of the output voltage V_(CC1) provided at the output nodeis programmable by a control signal CTL1.

The reference voltage generator 105 generates a reference voltageV_(REF), which is provided to a non-inverting input of the erroramplifier 101, in this embodiment. The regulation FET 102 includes asource electrically connected to a second voltage V₂, which can beinstance, a power high supply voltage, such as a battery voltage. Theregulation FET 102 further includes a gate electrically connected to anoutput of the error amplifier 101 and a drain electrically connected tothe LDO regulator's output. The feedback circuit 83 is electricallyconnected between the LDO regulator's output and a first voltage V₁, andprovides a feedback signal to an inverting input of the error amplifier101. The output capacitor 106 is electrically connected between theoutput of the LDO regulator 80 and the first voltage V₁. Additionally,the alternative discharge circuit 84 is electrically connected betweenthe output of the LDO regulator 80 and the first voltage V₁.

Although the LDO regulator 80 of FIG. 5A illustrates one example of anLDO regulator that includes an alternative discharge circuit, theteachings herein are applicable to LDO regulators implemented in a widevariety of ways.

In the illustrated configuration, the LDO regulator 80 is programmed bycontrolling a resistance of the feedback circuit 83 using the controlsignal CTL1. However, other configurations are possible. For example, anLDO regulator can be programmed in a variety of ways, such as byadjusting a resistance of a feedback resistor and/or a voltage of areference voltage to an error amplifier. As shown in FIG. 5A, thecontrol signal CTL1 is also provided to the alternative dischargecircuit 84, which can aid the alternative discharge circuit 84 indetermining the LDO regulator's target voltage, or steady-state voltageduring regulation.

A transient response of the LDO regulator 80 can depend on slewing rate,bandwidth, and a capacitance of the output capacitor 106.

When the output voltage V_(CC1) is programmed from a low voltage to ahigh voltage, the regulation FET 102 can be used to charge the outputcapacitor 106. Thus, LDO regulator 80 can achieve relatively fasttransient response for low voltage to high voltage transitions of theoutput voltage V_(CC1) by implementing the regulation FET 102 to besufficiently wide to charge the output capacitor 106 in accordance withtiming specifications.

However, when the output voltage V_(CC1) is programmed from a highvoltage to a low voltage, the regulation FET 102 can be turned off.Absent inclusion of the alternative discharge circuit 84, a transientresponse for high voltage to low voltage transitions of the outputvoltage V_(CC1) can be based on an RC time constant associated with aresistance of the feedback circuit 83 and a capacitance of the outputcapacitor 106.

Such an RC time constant may be unsuitable for applications in which theoutput voltage V_(CC1) is used to power RF electronics. For instance, toachieve stability in the presence of variations in loading (representedschematically by a resistor R_(L)), the output capacitor 106 can have arelatively large capacitance value. Additionally, to achieve lowquiescent current, the feedback circuit 83 can have a relatively largeresistance to reduce leakage current from the second voltage V₂ to thefirst voltage V₁ through the regulation FET 102 and the feedback circuit83. Thus, the RC time constant associated with the resistance of thefeedback circuit 83 and the capacitance of the output capacitor 106 canbe relatively large. In such a configuration, the LDO regulator may havean RC time constant of 50 μs or more, which may be insufficient to meeta settling time specification in mobile applications.

The illustrated LDO regulator 80 includes the alternative dischargecircuit 84, which activates to discharge the output capacitor 106 whenthe output voltage V_(CC1) is programmed from a high voltage to a lowvoltage.

In certain configurations, the alternative discharge circuit 84 includesa detection circuit that compares the output voltage V_(CC1) to a targetvoltage that is generated based on the control signal CTL1. Based oncomparing the output voltage V_(CC1) and the target voltage, thedetection circuit can selectively activate an electrical path throughthe alternative discharge circuit 84 from the LDO regulator's output tothe first voltage V₁ to discharge the output capacitor 106. In certainconfigurations, the electrical path through the alternative dischargecircuit 84 is activated when the voltage difference between the outputvoltage V_(CC1) and the target voltage is greater than a threshold ormargin voltage, and is deactivated when the voltage difference betweenthe output voltage V_(CC1) and the target voltage is less than thethreshold voltage.

Thus the alternative discharge circuit 84 can be used to discharge theoutput capacitor 106 when the output voltage V_(CC1) is relatively highand far from the target voltage. However, when the output voltageV_(CC1) and the target voltage are relatively close to one another, theelectrical path through the alternative discharge circuit 84 can bedeactivated. Configuring the LDO regulator 80 in this manner can aid inpreventing the alternative discharge circuit 84 from generatingovershoot, undershoot and/or glitch in the output voltage V_(CC1).

Additional details for the LDO regulator 80 can be as described earlier.

FIG. 5B is a schematic diagram of another embodiment of an LDO regulator90. The LDO regulator 90 of FIG. 5B is similar to the LDO regulator 80of FIG. 5A, except that the LDO regulator 90 of FIG. 5B includes adifferent configuration of output voltage programming.

In particular, the illustrated LDO regulator 90 includes a referencevoltage generator 105 that controls the voltage level of the referencevoltage V_(REF) based on the control signal CTL1. The voltage level ofthe reference voltage V_(REF) can be controlled to achieve a desiredoutput voltage V_(CC1), since the error amplifier 101 is connected withnegative feedback that operates to control the voltage levels of theerror amplifier's non-inverting and inverting inputs to be about equalto one another.

Additional details of the LDO regulator 90 can be as described earlier.

FIG. 6 is a schematic diagram of another embodiment of an LDO regulator100. The LDO regulator 100 includes a reference voltage generator 101, aregulation FET 102, and an output capacitor 106, which can be asdescribed earlier. The LDO regulator 100 further includes a feedbacknetwork 103 and an alternative discharge circuit 104. The LDO regulator100 of FIG. 6 illustrates one implementation of the LDO regulator 80 ofFIG. 5A. However, the LDO regulator 80 of FIG. 5A can be implemented inother ways.

The feedback circuit 103 includes a first resistor 107 and a secondresistor 108, which are electrically connected in series between theoutput of the LDO regulator 100 and the first voltage V₁. The first andsecond resistors 107, 108 operate as a voltage divider, and anintermediate node between the first and second resistors 107, 108 isused to provide a feedback signal to the inverting input of the erroramplifier 101. In the illustrated configuration, the first resistor 107is programmable based on the control signal CTL1. By changing theresistance of the first resistor 107, a ratio of the resistances of thefirst and second resistors 107, 108 can be controlled to achieve adesired output voltage V_(CC1). Although one implementation of afeedback circuit has been shown in FIG. 6, a feedback circuit for an LDOregulator can be implemented in other ways.

The illustrated alternative discharge circuit 104 includes a triggervoltage generation circuit 111, a comparator 112, a discharge FET 113,and a discharge control circuit 114.

The trigger voltage generation circuit 111 generates a trigger voltageV_(TRIGGER) for a first input of the comparator 112. The comparator 112compares the trigger voltage V_(TRIGGER) to the output voltage V_(CC1)received at a second input. The trigger voltage generation circuit 111includes a replica resistor 122, which is a replica of the firstresistor 107 of the feedback circuit 103. As shown in FIG. 6, thereplica resistor 122 is a programmable resistor controlled using thefirst control signal CTL1, and has a resistance that changes with theresistance of the first resistor 107. The trigger voltage generationcircuit 111 further includes a current source 121, which is configuredto generate a current that passes through the replica resistor 122. Avoltage drop of the replica resistor 122 changes in relation to the LDOregulator's target voltage.

The illustrated trigger voltage generation circuit 111 further includesa margin resistor 123, which is programmable using a margin signalMARGIN in this embodiment. The current generated by current source 121also flows through the margin resistor 123 to generate a margin voltage.The trigger voltage generation circuit 111 outputs a trigger voltageV_(TRIGGER) corresponding to a sum of a target voltage of the LDOregulator across the replica resistor 122 and a margin voltage acrossthe margin resistor 123. The resistance of the margin resistor 123 canbe programmed to control timing of when a discharge path through thealternative discharge circuit 104 is active.

The comparator 112 compares the trigger voltage V_(TRIGGER) generated bythe trigger voltage generation circuit 111 to the output voltage V_(CC1)of the LDO regulator 100. In the illustrated embodiment, when the outputvoltage V_(CC1) is greater than the trigger voltage V_(TRIGGER), thecomparator 112 turns on the discharge FET 113 to provide a low impedancedischarge path from the output of the LDO regulator 100 to the firstvoltage V₁. However, when the output voltage V_(CC1) is less than thetrigger voltage V_(TRIGGER), the comparator 112 turns off the dischargeFET 113, and the output voltage V_(CC1) is controlled based onregulation operations of the LDO regulator 100.

The resistance of the margin resistor 123 can be increased or decreasedto control the voltage level of the trigger voltage V_(TRIGGER). Sincethe trigger voltage V_(TRIGGER) determines when the electrical paththrough the alternative discharge circuit 104 is deactivated, theresistance of the margin resistor 123 controls timing of a hand-off ofcontrol of the output voltage V_(CC1) from the alternative dischargecircuit 104 to the LDO regulator's regulation loop. The margin resistor123 can be programmed with a resistance sufficient to prevent thealternative discharge circuit 104 from generating overshoot, undershootand/or glitch in the output voltage V_(CC1). The margin resistor 123 canalso be programmed to compensate for manufacturing and/or temperaturevariations.

The illustrated alternative discharge circuit 104 further includes thedischarge control circuit 114, which can be used to limit a maximumamount of current that the alternative discharge circuit 104 candischarge from the output capacitor 106. In one embodiment, thedischarge control circuit 114 includes a resistor that is in series withthe discharge FET 113. In one embodiment, the resistor is about 10Ω.Including the discharge control circuit 114 can help reduce ringing inthe output voltage V_(CC1). Furthermore, when the comparator 112 turnsoff the discharge FET 113, the discharge control circuit 114 can inhibitcharge present in the channel of the discharge FET 113 from flowing backinto the output capacitor 106. Thus, inclusion of the discharge controlcircuit 114 can enhance transient performance of the LDO regulator 100.

Additional details of the LDO regulator 100 can be similar to thosedescribed earlier.

FIG. 7A is a schematic diagram of one embodiment of a packaged module300. FIG. 7B is a schematic diagram of a cross-section of the packagedmodule 300 of FIG. 7A taken along the lines 7B-7B.

The packaged module 300 includes an IC or die 301, surface mountcomponents 303, wirebonds 308, a package substrate 320, andencapsulation structure 340. The package substrate 320 includes pads 306formed from conductors disposed therein. Additionally, the die 301includes pads 304, and the wirebonds 308 have been used to electricallyconnect the pads 304 of the die 301 to the pads 306 of the packagesubstrate 301.

As illustrated in FIGS. 7A and 7B, the die 301 includes an erroramplifier 101, a regulation FET 102, a feedback circuit 103, and analternative discharge circuit 104, which can be as described earlier. Inone embodiment, the die 301 further includes a voltage generator 105.

The packaging substrate 320 can be configured to receive a plurality ofcomponents such as the die 301 and the surface mount components 303,which can include, for example, surface mount capacitors and/orinductors. In one embodiment, the surface mount components 303 includean output capacitor 106.

As shown in FIG. 7B, the packaged module 300 is shown to include aplurality of contact pads 332 disposed on the side of the packagedmodule 300 opposite the side used to mount the die 301. Configuring thepackaged module 300 in this manner can aid in connecting the packagedmodule 300 to a circuit board such as a phone board of a wirelessdevice. The example contact pads 332 can be configured to provide RFsignals, bias signals, power low voltage(s) and/or power high voltage(s)to the die 301 and/or the surface mount components 303. As shown in FIG.7B, the electrically connections between the contact pads 332 and thedie 301 can be facilitated by connections 333 through the packagesubstrate 320. The connections 333 can represent electrical paths formedthrough the package substrate 320, such as connections associated withvias and conductors of a multilayer laminated package substrate.

In some embodiments, the packaged module 300 can also include one ormore packaging structures to, for example, provide protection and/orfacilitate handling of the packaged module 300. Such a packagingstructure can include overmold or encapsulation structure 340 formedover the packaging substrate 320 and the components and die(s) disposedthereon.

It will be understood that although the packaged module 300 is describedin the context of electrical connections based on wirebonds, one or morefeatures of the present disclosure can also be implemented in otherpackaging configurations, including, for example, flip-chipconfigurations.

FIG. 8A is a graph of one example of laboratory data of output voltageversus time for an LDO regulator. The graph includes a first plot 501 ofvoltage versus time for a high voltage to low voltage transition of anLDO regulator implemented without an alternative discharge circuit. Thegraph of FIG. 8A also includes a table of measurement data associatedwith the laboratory data. As shown in FIG. 8A, the LDO regulator canexhibit a relatively slow transient response for the high voltage to lowvoltage transition. In particular, a fall time of the LDO regulatorassociated with changing the programmable output voltage of the LDOregulator from a high voltage level to a low voltage level can berelatively poor.

FIG. 8B is a graph of another example of laboratory data of outputvoltage versus time for an LDO regulator. The graph includes a secondplot 502 of voltage versus time for a high voltage to low voltagetransition of an LDO regulator implemented with an alternative dischargecircuit. The graph of FIG. 8B also includes a table of measurement dataassociated with the laboratory data. As shown in FIG. 8B, the LDOregulator can exhibit a relatively fast transient response. Although oneexample of laboratory data is shown, results can differ based on a widevariety of factors, including, for example, application and/orimplementation.

Applications

Some of the embodiments described above have provided examples inconnection with wireless devices or mobile phones. However, theprinciples and advantages of the embodiments can be used for any othersystems or apparatus that have needs for LDO regulators.

Such LDO regulators can be implemented in various electronic devices.Examples of the electronic devices can include, but are not limited to,consumer electronic products, parts of the consumer electronic products,electronic test equipment, etc. Examples of the electronic devices canalso include, but are not limited to, memory chips, memory modules,circuits of optical networks or other communication networks, and diskdriver circuits. The consumer electronic products can include, but arenot limited to, a mobile phone, a telephone, a television, a computermonitor, a computer, a hand-held computer, a personal digital assistant(PDA), a microwave, a refrigerator, an automobile, a stereo system, acassette recorder or player, a DVD player, a CD player, a VCR, an MP3player, a radio, a camcorder, a camera, a digital camera, a portablememory chip, a washer, a dryer, a washer/dryer, a copier, a facsimilemachine, a scanner, a multi-functional peripheral device, a wrist watch,a clock, etc. Further, the electronic devices can include unfinishedproducts.

CONCLUSION

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense, as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” The word “coupled”, as generally usedherein, refers to two or more elements that may be either directlyconnected, or connected by way of one or more intermediate elements.Likewise, the word “connected”, as generally used herein, refers to twoor more elements that may be either directly connected, or connected byway of one or more intermediate elements. Additionally, the words“herein,” “above,” “below,” and words of similar import, when used inthis application, shall refer to this application as a whole and not toany particular portions of this application. Where the context permits,words in the above Detailed Description using the singular or pluralnumber may also include the plural or singular number respectively. Theword “or” in reference to a list of two or more items, that word coversall of the following interpretations of the word: any of the items inthe list, all of the items in the list, and any combination of the itemsin the list.

Moreover, conditional language used herein, such as, among others,“can,” “could,” “might,” “can,” “e.g.,” “for example,” “such as” and thelike, unless specifically stated otherwise, or otherwise understoodwithin the context as used, is generally intended to convey that certainembodiments include, while other embodiments do not include, certainfeatures, elements and/or states. Thus, such conditional language is notgenerally intended to imply that features, elements and/or states are inany way required for one or more embodiments or that one or moreembodiments necessarily include logic for deciding, with or withoutauthor input or prompting, whether these features, elements and/orstates are included or are to be performed in any particular embodiment.

The above detailed description of embodiments of the invention is notintended to be exhaustive or to limit the invention to the precise formdisclosed above. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. For example, whileprocesses or blocks are presented in a given order, alternativeembodiments may perform routines having steps, or employ systems havingblocks, in a different order, and some processes or blocks may bedeleted, moved, added, subdivided, combined, and/or modified. Each ofthese processes or blocks may be implemented in a variety of differentways. Also, while processes or blocks are at times shown as beingperformed in series, these processes or blocks may instead be performedin parallel, or may be performed at different times.

The teachings of the invention provided herein can be applied to othersystems, not necessarily the system described above. The elements andacts of the various embodiments described above can be combined toprovide further embodiments.

While certain embodiments of the inventions have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. Indeed, the novel methodsand systems described herein may be embodied in a variety of otherforms; furthermore, various omissions, substitutions and changes in theform of the methods and systems described herein may be made withoutdeparting from the spirit of the disclosure. The accompanying claims andtheir equivalents are intended to cover such forms or modifications aswould fall within the scope and spirit of the disclosure.

What is claimed is:
 1. A mobile device comprising: a transceiverconfigured to generate a radio frequency signal; a multi-stage poweramplifier configured to amplify the radio frequency signal, themulti-stage power amplifier including a driver stage and an outputstage; and a power management system including an envelope trackerconfigured to control a supply voltage of the output stage in relationto an envelope of the radio frequency signal, and a programmable lowdropout regulator including an output configured to provide aprogrammable supply voltage to the driver stage, the programmable lowdropout regulator including an output capacitor electrically connectedto the output, a regulation transistor having a drain electricallyconnected to the output, and an alternative discharge circuit configuredto activate an electrical path to discharge the output capacitor inresponse to programming the programmable supply voltage from a highvoltage level to a low voltage level, the alternative discharge circuitincluding a comparator configured to activate the electrical path basedon comparing the programmable supply voltage to a trigger voltage. 2.The mobile device of claim 1 wherein the regulation transistor isconfigured to charge the output capacitor in response to programming theprogrammable supply voltage from the low voltage level to the highvoltage level.
 3. The mobile device of claim 2 wherein the alternativedischarge circuit is electrically connected between the output of theprogrammable low dropout regulator and a ground voltage.
 4. The mobiledevice of claim 1 wherein the alternative discharge circuit furtherincludes a discharge transistor in the electrical path and including agate controlled by an output of the comparator.
 5. The mobile device ofclaim 1 wherein the programmable low dropout regulator is configured toreceive a control signal operable to control a programmed voltage levelof the programmable supply voltage, the alternative discharge circuitconfigured to adjust a voltage level of the trigger voltage based on thecontrol signal.
 6. The mobile device of claim 1 wherein the programmablelow dropout regulator further includes an error amplifier configured tocontrol a gate of the regulation transistor.
 7. The mobile device ofclaim 1 further comprising an antenna configured to transmit anamplified radio frequency signal from the multi-stage power amplifier.8. A power amplifier system comprising: a multi-stage power amplifierconfigured to amplify a radio frequency signal, the multi-stage poweramplifier including a driver stage and an output stage; an envelopetracker configured to control a supply voltage of the output stage inrelation to an envelope of the radio frequency signal; and aprogrammable low dropout regulator including an output configured toprovide a programmable supply voltage to the driver stage, theprogrammable low dropout regulator including an output capacitorelectrically connected to the output, a regulation transistor having adrain electrically connected to the output, and an alternative dischargecircuit configured to activate an electrical path to discharge theoutput capacitor in response to programming the programmable supplyvoltage from a high voltage level to a low voltage level, thealternative discharge circuit including a comparator configured toactivate the electrical path based on comparing the programmable supplyvoltage to a trigger voltage.
 9. The power amplifier system of claim 8wherein the regulation transistor is configured to charge the outputcapacitor in response to programming the programmable supply voltagefrom the low voltage level to the high voltage level.
 10. The poweramplifier system of claim 8 wherein the alternative discharge circuit iselectrically connected between the output of the programmable lowdropout regulator and a ground voltage.
 11. The power amplifier systemof claim 8 wherein the alternative discharge circuit further includes adischarge transistor in the electrical path and including a gatecontrolled by an output of the comparator.
 12. The power amplifiersystem of claim 8 wherein the programmable low dropout regulator isconfigured to receive a control signal operable to control a programmedvoltage level of the programmable supply voltage, the alternativedischarge circuit configured to adjust a voltage level of the triggervoltage based on the control signal.
 13. The power amplifier system ofclaim 8 wherein the programmable low dropout regulator further includesan error amplifier configured to control a gate of the regulationtransistor.
 14. A method of signal amplification in a mobile device, themethod comprising: amplifying a radio frequency signal using a driverstage and an output stage of a multi-stage power amplifier; controllinga supply voltage of the output stage in relation to an envelope of theradio frequency signal using an envelope tracker; and providing thedriver stage with a programmable supply voltage from an output of aprogrammable low dropout regulator, including stabilizing theprogrammable supply voltage using an output capacitor electricallyconnected to the output, regulating the programmable supply voltageusing a regulation transistor having a drain electrically connected tothe output, activating an electrical path based on comparing theprogrammable supply voltage to a trigger voltage using a comparator ofan alternative discharge circuit, and discharging the output capacitorusing the electrical path in response to programming the programmablesupply voltage from a high voltage level to a low voltage level.
 15. Themethod of claim 14 further comprising charging the output capacitorusing the regulation transistor in response to programming the outputvoltage from the low voltage level to the high voltage level.
 16. Themethod of claim 14 further comprising programming the voltage level ofthe programmable supply voltage using a control signal, and adjusting avoltage level of the trigger voltage based on the control signal. 17.The method of claim 14 wherein selectively activating the electricalpath through the alternative discharge circuit includes controlling agate of a discharge transistor based on the comparison, and limiting anamount of current through the alternative discharge circuit using adischarge control circuit that is in series with the dischargetransistor.
 18. The mobile device of claim 6 wherein the error amplifierincludes a first input configured to receive the programmable supplyvoltage and a second input configured to receive a reference voltage,the alternative discharge circuit configured to control the triggervoltage separately from the reference voltage.
 19. The mobile device ofclaim 18 wherein the alternative discharge circuit is configured tocontrol a voltage level of the trigger voltage based on a margin signalfor adjusting timing of a hand-off of control of the programmable supplyvoltage from the alternative discharge circuit to the error amplifier.20. The mobile device of claim 18 wherein the alternative dischargecircuit is configured to control a voltage level of the trigger voltagebased on a programmed voltage level of the programmable supply voltage.